Magnetic sensors are widely used in modern systems to measure or detect physical parameters including but not limited to magnetic field strength, current, position, motion, orientation, and so forth. There are many different types of sensors in the prior art for measuring magnetic field, but these sensors have limitations that are well known in the art, such as, excessive size, inadequate sensitivity and/or dynamic range, cost, reliability and other factors.
Hence, there is a need for improved magnetic sensors, especially sensors that can be easily integrated with semiconductor devices and integrated circuits and manufacturing methods thereof.
Magnetic tunnel junction (MTJ) sensors have the advantages of high sensitivity, small size, low cost, and low power consumption. Although MTJ devices are compatible with standard semiconductor fabrication processes, methods for building high performance MTJ linear magnetic field sensors have not been adequately developed. In particular, performance issues due to temperature dependence and hysteresis are not easy to control.
Magnetic field sensors may be constructed from a single magnetoresistive element, but in practice it is advantageous to configure several magnetoresistive elements into a Wheatstone bridge in order to eliminate offset, increase sensitivity, and provide some level of temperature compensation. Although bridge configurations do improve temperature compensation, the inherent temperature dependence of the magnetoresistance and magnetic properties of the sensor are not completely suppressed. For high accuracy, it is desirable to calibrate the sensitivity during operation, and an on-chip calibration coil that produces a known magnetic field along the sensitive direction of the sensor is often provided for this purpose. Calibration is often performed by periodically applying a low amplitude current pulse sequence to the calibration coil, which provides a known magnetic field pulse sequence from which the sensitivity of the magnetoresistive sensor may be determined during operation of the magnetometer.
Because magnetoresistive sensors are composed of ferromagnetic sensing elements, the sensor response is subject to nonlinearities, offset, and hysteresis due to the formation and motion of domain walls within the sensor elements or other components, such as magnetic shields and flux concentrators. To overcome this issue, high performance magnetoresistive sensors are often provided with another coil, orthogonal to the calibration coil that is used to periodically saturate the sensor elements and sweep out magnetic domains. This is referred to as a set/reset coil.
The presence of both the calibration and set/reset coils adds complexity to magnetoresistive sensor fabrication by increasing the number of process steps required to manufacture the sensor, and it increases the size of the sensor die by requiring more contact pads and to accommodate the geometrical constraints required to produce the orthogonal calibration and set/reset fields.
Magnetoresistive sensors without a calibration coil are possible. A disadvantage of this approach is the fact that the sensitivity of the sensor cannot be measured by electrical means. That is, if the magnetoresistive sensor does not have a calibration coil, the response of the sensor cannot be monitored and analyzed for sensitivity. Moreover, implementing a standard self-test in the sensor is cumbersome.
The magnetic field that is generated by a line current decreases inversely proportionally with the distance from the line. Power optimization indicates that the distance between the sensor and the calibration coil, and the distance between the sensor and the reset coil should be as small as possible. Ideally both coils should be located as close as possible to the sensor. This is however physically impossible.